Here's a Riddle
 

When traveling is it......
A) Better to fill both tanks with fuel for less stops and carrying the extra
weight or
B) Lightening the load and making more stops.

I realize this is personal opinion but I wounder about this quite often and no there is no trailer involved.

K. I have seen it said that 100 lbs. of weight can cost .1 mpg. But I think that is on a little car. My sons psd will run really well and it don't care a lot if it's loaded or not. Gas is about 6.25 lbs a gallon.

I would fill both tanks. They say condensation will build if you don't. And use this to purchase fuel at lower cost instead of fueling up at higher priced places. For example gas cost less in my home town and with two tanks I can make Denver and skip all the high priced fuel stops up on I 70.

I still make stops though. I don't do it to save time I do it to save money. A 4-5 thousand pound truck I don't see a 100 lbs or so of fuel making any difference.

Jack I've had my fill (no pun intended) of buildup in the rear tank

I fill both tanks. the newer superdutys have a 38 gal tank, and I fill the 2002 to the top. by the time you add the small amout of weight, you won't notice it in mileage loss, a head wind will be worse for burning fuel. my 86, and 82 have only 1 tank, and it sucks only being able to run 3 hrs between fill ups. just my .02

I'd say fill both tanks.

i fill both tanks and go 600 miles between fill ups

Don't think she'll go 600 Ronnie twin 18 gallon gas tanks and I think the best I got at 70mph was around 160 miles

wow. and you need to ask if you should fill both tanks? DUH.you have something seriously wrong with your truck. my 90 f 250 w/460 gets 440 miles from both tanks with a load on it~~~~~~~~ the 04 pete w/315 cummings gets 400 mi on 40 gal, and weighs 18,000 lbs empty~~~~~~

Depends on where you're going! Fill one if you're going to an area with lower prices, so that you can fill the second cheaper, and fill both tanks if you know that prices are going to be higher. Me, I need coffee more often than once every three hours, I stop for coffee much more often than I stop for gas! Of course, that also means more "necessary" stops! :-X04

When I had the choice (the old '92 F250 w/2 tanks), I'd fill both when the price was right when I wasn't towing (19MPG) - if towing, I had no choice but to fill both, whatever the cost). Total of 36 gal of diesel.

With the new truck, as Charlie says, 38 gal single tank. I fill it all the time!

back when I had 2 tanks, I'd fill them both before I left just so I had the option of keepin' on keepin' on if the prices I saw out on the road were high. If I saw a lower price, I'd stop and top both tanks as needed.
All this goes sideways if I HAD to be somewhere RIGHT NOW- I'd fill both, run them dry, fill 'em both again, no matter the cost.